The use of an uninterruptible power system (UPS) to provide power to a critical load is well known. Known uninterruptible power systems include on-line UPS's and off-line UPS's. On-line UPS's provide conditioned AC power as well as back-up AC power upon interruption of a primary source of AC power. Off-line UPS's typically do not provide conditioning of input AC power, but do provide back-up AC power upon interruption of the primary AC power source. FIG. 1 shows a block diagram of one type of on-line UPS 10. Other on-line UPS's are described in U.S. Pat. Nos. 5,982,652, and 5,686,768, both of which are incorporated herein by reference. On-line UPS's of the type described in the referenced patents are available from American Power Conversion Corporation, West Kingston, R.I. under the trade names Symmetra and Silcon. The UPS 10 of FIG. 1 includes an input circuit breaker/filter 12, a rectifier 14, a control switch 15, a controller 16, a battery 18, an inverter 20, an isolation transformer 22, and a bypass switch 23. The UPS also includes an input 24 for coupling to an AC power source, and an outlet 26 for coupling to a load.
The UPS 10 operates as follows. The circuit breaker/filter 12 receives input AC power from the AC power source through the input, filters the input AC power and provides filtered AC power to the rectifier 14. The rectifier rectifies the input voltage. The control switch 15 receives the rectified power and also receives DC power from the battery 18. The controller 16 determines whether the power available from the rectifier is within predetermined tolerances, and if so, controls the control switch to provide the power from the rectifier to the inverter 20. If the power from the rectifier is not within the predetermined tolerances, which may occur because of “brown out” or “black out” conditions, or due to power surges, then the controller controls the control switch to provide the DC power from the battery to the inverter 20.
The inverter 20 of the UPS 10 receives DC power and converts the DC power to AC power and regulates the AC power to predetermined specifications. The inverter 20 provides the regulated AC power to the isolation transformer 22. The isolation transformer is used to increase or decrease the voltage of the AC power from the inverter and to provide isolation between a load and the UPS. The isolation transformer is typically an optional device, the use of which is typically dependent on UPS output power specifications. Depending on the capacity of the battery and the power requirements of the load, the UPS 10 can provide power to the load during brief power source dropouts or for extended power outages. The bypass switch 23 is used to provide a bypass of UPS circuitry to provide the input power directly to the output. The bypass switch may be controlled by the controller 16 to provide bypass of the UPS circuitry upon a failure condition of the UPS.
To provide further power redundancy, it is known to supply power to a bypass switch of a UPS from a second source of AC power as shown in FIG. 2. One problem with this approach is that in bypass mode, the load receives unconditioned power and the source and the load must be able to handle transients that may occur when the load is transferred to the second AC source. In some systems, to minimize transients, the first and second AC sources are required to be substantially synchronous and have substantially the same voltage. To at least partially overcome these problems, a second UPS may be installed in line with the second AC source, but such a solution can be quite expensive since two UPS's are used.